Internal fixation devices or fixators which serve to rigidly couple a vertebra in the spinal column to one or more other vertebrae are known. In general they involve screws such as the so-called "Schanz" screws which are inserted into vertebrae and extend out more or less normally to the back of the patient, support rods which extend roughly parallel to the spine, and clamps or couplings which connect the screws with the support rod. Such fixators are described, for example, in the Synthes Bulletin of the Association for the Study of Internal Fixation, No. 70, dated March, 1986.
In systems according to the bulletin referred to, which are described more fully below, the Schanz screws are held by a kind of hook which presses the shaft of the screw against a clamp housing under the pressure of a nut turning on a screw on which the hook is mounted. This structure is difficult to work with. Moreover, the Schanz screw is not retained as rigidly as might be desired.